When the presence of CTC positivity was assessed based on completely different transcripts, as in the case of singleplex RT-qPCR for CK-19 versus AdnaTest BreastCancerTM, or multiplex RT-qPCR versus AdnaTest BreastCancerTM, there were discrepancies when the number of CTC was low, as in early breast cancer. Here, KRT19 is linked to breast cancer.